How to get notified when droplet reboots and when droplet finishes boot?

Question

I found this answer https://stackoverflow.com/a/35456310/80353 and it recommends either API or using user_data which actually is cloud-init underneath.

I can think of several ways to possibly get notified that a server is up:

1. detect droplet status via API

   I notice that the status never changes during reboot so I guess this is out.

2. using DigitalOcean native monitoring agent

   The monitoring agent seems to only cover resource utilisation. No alert when the server is being rebooted or finishes booting up

3. using cloud-init

   This answer https://stackoverflow.com/a/35456310/80353 I mentioned earlier uses wget to send signals out. I can possibly use wget for every time the droplet finishes booting up using bootcmd in cloud-init. But not for reboot.

   There's also the issue of how to ensure the wget request from the right DigitalOcean droplet can correctly identify itself to my server.

Any advice on how to accomplish getting notifications at my server whenever a droplet reboots or finishes booting up?